This invention relates to shaped frictional elements, particularly brake linings, disc pads and clutch facings and to a method of manufacturing the same which involves the use of liquid phenolic resins and catalyst set at room temperature.
In the conventional process for the manufacture of shaped frictional elements fillers such as asbestos fibres, barytes, magnesia brass or zinc filings or turnings are mixed with a binder comprising a powdered phenol formaldehyde novolak/hexamine blend and hot pressed in a mould. The action of the heat from the mould is first to fuse the binder and then to cure it.
It is, however, a disadvantage of this process that noxious gases and fumes of ammonia, formaldehyde and free phenol are evolved during the curing process. The workers need to wear gloves in order to handle the ejected cured mouldings and generally the conditions of work are very unpleasant.
It is a further disadvantage of the conventional process for the production of shaped frictional products that the expenditure of considerable energy is necessary to effect the fusion and setting of the binder.
It is known to accelerate the cure of these materials by the addition of acids or anhydrides. Many frictional materials, such as asbestos, are, however, alkaline while many of the metals included in frictional materials, while not alkaline, nevertheless react with acids. In this way much, if not all, of the accelerating effect of acids and anhydrides is vitiated when these materials are employed in an attempt to accelerate phenolic resins in bonding frictional compositions. In addition, the presence of acids is undesirable for a number of other reasons, such as their corrosive effect on metals, both in manufacture and use, and the additional handling problems, environmental effects and health risks which their use entails.
The present invention seeks to improve these aspects of production by eliminating the heat curing steps during formation of the shaped frictional materials. The reduction in energy brought about by cold curing is a substantial economic advantage of the system but there is also a reduction in wear on the moulding tool and the need for extensive ventilation is eliminated.
We have now found that shaped frictional articles can be produced quickly and economically at room temperature, without the hazards and inconveniences associated with heated moulds, by a process in which an aqueous highly alkaline phenol-formaldehyde condensation product is employed as a binder, said binder is intimately mixed with a quantity of frictional filler and subsequently with a second quantity of frictional filler which has previously been mixed with a quantity of a liquid ester sufficient to effect cure of the phenolic resin.
These compositions set at room temperature and therefore demand less energy than the heat cured systems previously used. The low curing temperature results in the substantial elimination of noxious fumes and the hazards associated with the handling of heated moulds. Being free from acidic materials, corrosion problems are greatly reduced or eliminated. In addition, because they are catalyzed by alkalies, they set in the presence of alkaline fillers such as asbestos.